Affirming the consequent

Affirming the consequent meme from Drace

The antecedent in an indicative conditional is claimed to be true because the consequent is true. Affirming the Consequent Does not Confirm the Antecedent.

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Description

Affirming the consequent

The antecedent in an indicative conditional is claimed to be true because the consequent is true. Affirming the Consequent Does not Confirm the Antecedent.

This fallacy might be seen as a flawed (invalid!) attempt to use the modus ponens argument form. Recall that one of the premises in modus ponens affirms the antecedent of the hypothetical premise. In effect, with modus ponens, the antecedent necessitates the consequent. In the fallacious examples below, however, the consequent is affirmed instead of the antecedent.

Names

Affirming the consequent, converse error, fallacy of the converse, or confusion of necessity and sufficiency

Types

Deductive Argument, Formal Argument

Deductive Logic or formal Fallacies: These are fallacies that arise from errors in the form of the argument. For example an error in a mathematical proof or logical proof like modus ponens.

logical Form

If A, then B; B, therefore A

Weakness

One way to demonstrate the invalidity of this argument form is with a counter example with true premises but an obviously false conclusion.

Examples

Discord Examples:

1. 1

  If earth is a sphere with satelites(A) then we see should see satelites. (B) we have seen satelites. (B) therefor the earth is a sphere. (A)

1. 2

  if earth is a sphere (A) then we would see boats disappear bottom up. (B) we see boats disappear bottom up. (B) therefor the earth is a sphere. (A)

1. 3

  If the earth is a sphere (A) then we would see shadows like erastothenes saw. (B) We saw shadows like erastothenes saw. (B) therefore the earth is a sphere. (A)

in whichever case there are other potential causes for the effects falsely attributed to sphericity, and with the defect in form none of these statements stand logically or can get anyone any closer to the truth.

if they dont get it example

  If you are super gay (A) then we would see you acting like this. (B) we see you act like this. (B) therefore you are super gay. (A)

in this hypothetical example he might not be gay at all, he might simply have feminine traits and low testasterone production or he might just be better looking then the guy making the argument and so arguer is acting on his inesecurity trying to make himself feel better, regardless you cant assume A is true just because you confirmed B...

make sense?

Other examples

1. 1

  If Bill Gates owns Fort Knox, then he is rich. Bill Gates is rich. Therefore, Bill Gates owns Fort Knox.

Owning Fort Knox is not the only way to be rich. Any number of other ways to be rich exist.

However, one can affirm with certainty that "if someone is not rich" (non-Q), then "this person does not own Fort Knox" (non-P). This is the contrapositive of the first statement, and it must be true if and only if the original statement is true.

1. 2

Here is another useful, obviously-fallacious example, but one that does not require familiarity with who Bill Gates is and what Fort Knox is:

   If an animal is a dog, then it has four legs.
   My cat has four legs.
   Therefore, my cat is a dog.

Here, it is immediately intuitive that any number of other antecedents ("If an animal is a deer...", "If an animal is an elephant...", "If an animal is a moose...", etc.) can give rise to the consequent ("then it has four legs"), and that it is preposterous to suppose that having four legs must imply that the animal is a dog and nothing else. This is useful as a teaching example since most people can immediately recognize that the conclusion reached must be wrong (intuitively, a cat cannot be a dog), and that the method by which it was reached must therefore be fallacious.

1. 3

Arguments of the same form can sometimes seem superficially convincing, as in the following example:

   If Brian had been thrown off the top of the Eiffel Tower, then he would be dead.
   Brian is dead.
   Therefore, Brian was thrown off the top of the Eiffel Tower.

Being thrown off the top of the Eiffel Tower is not the only cause of death, since there exist numerous different causes of death.

Affirming the consequent is commonly used in rationalization, and thus appears as a coping mechanism in some people.

1. 4

In Catch-22, the chaplain is interrogated for supposedly being "Washington Irving"/"Irving Washington", who has been blocking out large portions of soldiers' letters home. The colonel has found such a letter, but with the Chaplain's name signed.

   "You can read, though, can't you?" the colonel persevered sarcastically. "The author signed his name."
   "That's my name there."
   "Then you wrote it. Q.E.D."

P in this case is 'The chaplain signs his own name', and Q 'The chaplain's name is written'. The chaplain's name may be written, but he did not necessarily write it, as the colonel falsely concludes. (weakness)

One way to demonstrate the invalidity of this argument form is with a counterexample with true premises but an obviously false conclusion.

1. 5

  Premise 1: If I’m cleaning the kitchen, then I’m not reading my book. Premise 2: I’m not reading my book. Conclusion: Thus, I’m cleaning the kitchen.

This reasoning is defective; think about it. The consequent cannot necessitate the antecedent. Not being engaged in reading the book does not, by necessity, infer that I am cleaning the kitchen. (Maybe I’m sleeping or out for a run.)

Extra information

Affirming the consequent is a formal fallacy of taking a true conditional statement (e.g., "If the lamp were broken, then the room would be dark,") and invalidly inferring its converse ("The room is dark, so the lamp is broken,") even though the converse may not be true. This arises when a consequent ("the room would be dark") has more than one other possible antecedents (for example, "the lamp is not plugged in" or "the lamp is in working order, but is switched off").

  The opposite statement, denying the consequent, is a valid form of argument. or modus tollens

Ref rulings

-Objection Stands:

The antecedent in an indicative conditional is claimed to be true because the consequent is true. that does not prove that it is true.

please reformulate the argument without this fallacy or concede and move on. failure to do either and you'll forfeit the debate.

-Objection Removed:

The antecedent in an indicative conditional is claimed to be true because the consequent is true. that does not prove that it is true. there was no affirming the consequence here so objection removed

please reformulate the counter argument without the fallacy fallacy variation or concede and move on. failure to do either and you'll forfeit the debate.

This page was written by Drace.